This invention relates to apparatus for capturing and retrieving objects, such as satellites, which are deployed in space.
In order to service an orbiting satellite from a spacecraft, such as America's space shuttle, it is necessary to provide apparatus capable of capturing and retrieving the satellite. In instances where the satellite does not have a cooperative docking interface, this is best accomplished by launching a grappling type device from the spacecraft toward the satellite. Since many satellites rotate about a predetermined satellite axis when in orbit, or may be tumbling or nutating, the retrieval apparatus must have the ability to bring such motion to a halt in this hostile environment. Furthermore, the retrieval apparatus should be capable of retrieving satellites of different size, mass and shape without causing damage to the satellites.
A satellite recovery system which addresses the basic requirements mentioned above is described in a paper entitled "Space Bola, A Satellite Retrieval System". This paper was presented by A. E. Wudell, D. A. Lewis and G. W. Smith of the Martin Company, at the Fourth Space Congress held at Cocoa Beach Fla. on Apr. 3-6, 1967.
The Wudell et al paper describes a grappling device which is guided to a satellite target and uses inflatable arms as the capturing mechanism. On contact, the arms are deflated and driven by their momentum or by solid propellant charges to enwrap the target. Small Velcro pads on the ends of the arms allow a lockup between the arms. Either a passive or active grappling device is proposed, depending on the dynamic characteristics of the target vehicle to be captured.
Although such a satellite recovery device, hereinafter referred to as the Martin device, bears a similarity to that disclosed and claimed herein, there are important differences in their construction, weight, size, strength, complexity, usefulness, target safety features, spin breaking mechanisms, recoverability, guidance mechanisms, effectiveness, reusability and energy efficiency. These differences will become apparent as this description of the two devices proceeds.
The Martin device, for example, has either two or three arms, hollow rubber tubes unrolled from the grappling unit and inflated by compressed air. Each arm preferrably has two inflatable air chambers, one to extend the arm, and one to cause encirclement of the target. The Velcro pads at the ends of the arms are very small (5.5 inches by 1.5 inches). Each arm also requires a vent port and possibly small propellant charges, remotely controlled, to aid the encirclement of the target. Each arm is unrolled by motor driven rollers, then inflated. The grappling unit must therefore include compressed air tanks, radio control devices and batteries.
For proper operation, it appears that the arms in the Martin device must unroll to the precise length needed to encircle a target, and the small Velcro pads must overlap with at least fifty percent accuracy. Therefore, the target diameter must be known in advance within one inch on a five foot target and the arm rollers must be capable of such accuracy. Clearly this is a very complex and expensive mechanism.
While no size is mentioned, a five foot target would appear to need three rolls of rubber arms at least eight feet long, coiled. No launching mechanism size is specified.
The strength of the inflatable arms of the Martin device is suspect when dealing with one ton targets. Target mass could easily be too much for inflatable arms and four square inches of Velcro pad to handle.
The Martin device utilizes one grappling arm configuration to capture spheres (three arms) and a different configuration to capture cylinders (two arms). A prior knowledge of the shape of the target satellite is therefore required. Also, as previously mentioned, the size of the target becomes critical and must be known within three inches in 15.7 feet of circumference.
If target spin or tumbling is to be cancelled, an attitude stabilization module must be added to the grappling unit, containing attitude stabilization thrusters, an onboard radar (or remote guidance via a television camera), fuel tanks and additional batteries. It would seem that the attitude stabilization module should always be available in the spacecraft in case the target has or acquires an unexpected spin. This additional 140 pound module required to accomplish spin braking is extremely complex and reduces the reliability of the retrieval system. It is in essence a guided rocket subject to numerous failures. Running it out of fuel or power renders it completely useless. Due to the tiny diameter of the unit, spin braking of large objects will be very slow and wasteful of fuel, if not impossible.
The Martin device weighs in the order of 200 pounds; 60 pounds for the passive grappling module and 140 pounds for the attitude stabilization module. These weight figures may even be conservative due to the complexity of this device. Adding this 200 pounds of weight to a spacecraft, such as the space shuttle, requires a significant increase in reaction fuel for the spacecraft to achieve a matching orbit.
Test results of the Martin device indicated that it required the impact of a fifteen foot drop to obtain ninety percent effective catches. One target in ten would need to be hit twice with that force. Also, the target must be capable of withstanding impact with a two hundred pound mass.
The Martin device uses power every second of its operation: for stabilization, radio contact, remote controls and a television link. Servicing the device would require three separate power sources: charged batteries, reaction fuel replacement, and recharged air tanks. Due to the complexity of the device, a complete operational checkout would seem to be prudent before relaunch.